PENTAGON � U.S. President-elect Donald Trump will formally announce that he has chosen retired General James Mad Dog Mattis for secretary of defense in a speech in North Carolina on Tuesday.

Mattis has served as the head of U.S. Central Command, which carries out U.S. operations in the Middle East, and the Supreme Allied Commander of NATO forces.

Michael O'Hanlon, a senior defense expert at the Brookings Institution, calls Mattis one of the best read, best informed and most experienced generals of his generation.

He's really almost thought of as a scholar as much as a general for his love of history, and strategy and even philosophy, and [he's] one of the brightest guys I've met in or out of uniform, O'Hanlon told VOA Monday.

His tenacity against enemy forces in Afghanistan and Iraq earned him both the Mad Dog nickname and respect from his military peers.

"He's a soldier's soldier; a Marine's Marine, General John Nicholson, the commander of International Forces in Afghanistan, told reporters at the Pentagon Friday. He's a very inspirational leader. I know he inspired me."

O'Hanlon said he believes Mattis' background and knowledge has made him more practical than most on the potential for military power and its limitations, and he told VOA he is encouraged by the pick.

It's actually good that someone with Mattis' battlefield credibility and experience is advising Donald Trump because Donald Trump will listen to that person, I believe, and will be more inclined to hear an argument for restraint in the use of force from someone like General Mattis, he said.

Before Mattis can be confirmed, Congress will have to pass an exception to a law that prohibits generals from serving as secretary of defense until they've been retired for seven years. Mattis has been retired for just three.

The law was written in 1947 and was meant to create space between military service and civilian directorship of the department. The retirement period was initially a 10-year duration, but Congress changed it to a seven-year retirement period in 2008, taking into account the fact that most generals retire in their late 50s or early 60s.

Congress has made one exception to the law, allowing World War II-era General George Marshall to serve as secretary of defense in 1950.

O'Hanlon said the law is one that has never been fully supported," because many who approved the exception to the law in 1950 helped create the law just a few years prior.

Trump made the first unofficial announcement of his pick while appearing at a post-election victory rally in Cincinnati, Ohio Thursday night.